Thanks Alan!Here's a photo of a vintage Oregon Universal Depth Gauge Part No. 540 identical to this one of yours that was sold by somebody on eBay. According to that tool's original included instructions, "The numbers at the dial screw represent five thousandths graduations. They are accurate from the base starting point; each individual will soon learn to set the gauge at the best point suited for his own personal needs." Hope this info is helpful for you or anybody else wishing to use this tool.
Welcome to A.S.!
Philbert